1. Field of the Invention
The present invention relates to a method for direct or indirect application of a liquid or viscous medium onto a moving material web, specifically a paper or cardboard web.
2. Description of the Related Art
In the paper producing industry, coating mediums are applied in the thickness range of one micrometer when applying coating or glue. The coating mediums take the form of glue, starch substance, pigment medium or conventional coating ink. It is very important that the layers applied to the paper web display a uniform profile, both in cross direction and in longitudinal direction in order to meet the high quality requirements put on paper by the paper manufacturer's customers and end users. A multitude of factors influence the thickness and quality of the applied layers. It is therefore easily understandable that, with conventional paper web widths of 10 m or wider, conventional machine speeds of up to 2000 m/min. or higher, and the associated large machinery required for paper production, disturbing factors may arise which would have a negative influence upon the coating result. These factors would include temperature related deformations of machinery components, mechanical wear and tear of doctoring elements, fluctuations of consistency of the coating medium, contaminations, etc. In order to gain control over these disturbing influences, modern paper production lines are equipped with sophisticated electronics which conduct measurements at many different locations within the production sequence. On the basis of these measurements, a control center, usually a process computer, triggers suitable control elements through which a targeted control can be exercised over the individual operating parameters of the machines. For example, it is quite common to measure the actual cross profile of the applied coating layer and, in the event of unevenness in the coating profile, to regulate the contact pressure and/or the angle of attack of a doctoring element by adjusting screws or hydraulic actuators.
All previously known solutions for elimination of defects in the coating result, that is, elimination of fluctuations from a desired coating result, have in common that, depending on the type of adjustment, one or more operating parameters are adjusted gradually relative to the observed defect until the defect in the coating result is no longer observed. The operating parameter or parameters will then remain at the newly adjusted values until possibly another defect is observed which would make necessary further adjustments of the operating parameter or parameters. This method is effective and corresponds with well known principles of control engineering.
Disturbing factors occasionally arise in a production run whose negative effects upon the coating result cannot be eliminated by the continuous adjustment of one or several operating parameters. If dirt particles, clumps of coating medium, fluff, dust particles or pulp fibers from the paper web adhere to exposed locations on an applicator unit in a machine for paper production, for example a nozzle gap of a free jet applicator, the resulting impairments of the coating result cannot be corrected easily by gradual adjustments of appropriate operating parameters. Specifically, this is often not possible in a relatively short time period, which is why large amounts of reject paper must be expected.